Many processes such as the corrosion of metals, the charging of a battery or reactions in electrocatalysis are prone to local activity changes of reactive species. In such complex systems, these changes often define the macroscopic behavior. However, a clear experimental deduction of individual reactions on the overall current signal appears to be very difficult in many cases. Whereas the influence of the local ion activities near the surface, exemplified for the Hydrogen Oxidation Reaction (HOR) / Hydrogen Evolution Reaction (HER) on Pt in H₂-saturated solutions has been discussed previously [1], it could be shown that for fast reactions, thermodynamic modeling predictions agree well with measured cyclic voltammograms [1, 2]. Further it was demonstrated that even in a weakly buffered solution (up to 10 mM total buffer concentration), the analytical equation not only reflects the experimental current-voltage characteristics but is also capable to deliver the local distribution of all contributing ions in the diffusion zone.

This work critically reviews the original modeling approach [1], illustrates its limitations and presents recent developments towards a more general model by including the local enrichment of buffer species and (slow) reaction kinetics, depending on the actual conditions at the electrode surface and in the bulk solution. Obtained results will be compared to recent progress in the field [3, 4], showing that the influence of local activity changes near the surface are likely to affect the behavior of the macroscopic system. This becomes particularly relevant for local electrochemical processes such as either atmospheric corrosion, the dissolution of a cathodic protection layer (e.g. Zn coated steel) or other reactions that consume/produce a large amount of protons.

While the modeling approach presented in this work cannot be directly used to derive the properties of a complex electrochemical system with a large multitude of reactive species and reaction steps, it remains helpful to deduce the influence of individual components on the macroscopic system.

References

[1]           M. Auinger, I. Katsounaros, J. C. Meier, S. O. Klemm, P. U. Biedermann,  A. A. Topalov, M. Rohwerder, K. J. J. Mayrhofer, Phys. Chem. Chem. Phys. 13 (2011) 16384-16394.

[2]           I. Katsounaros, J. C. Meier, S. O. Klemm, A. A. Topalov, P. U. Biedermann, M. Auinger, K. J. J. Mayrhofer, Electrochem. Commun. 13 (2011) 634.

[3]           D. Strmcnik, M. Uchimura, C. Wang, R. Subbaraman, N. Danilovic, V. van der Vliet, A. P. Paulikas, V. R. Stamenkovic, N. M. Markovic, Nat. Chem. 5 (2013) 300.

 [4]          M. D. Arce, H. L. Bonazza, J. L. Fernandez, Electrochim. Acta 107 (2013) 248.